1. Field of the Invention
The invention relates to luminescent screens for flat image display devices and somewhat more particularly to such screens wherein the individual luminescent dots are separated from one another by a contrasting border layer.
2. Prior Art
Flat image display devices functioning in accordance with the principles of gas discharge displays, as plasma panels or plasma displays are known, for example see German Offenlegungsschrift No. 24 12 869 (generally corresponding to U.S. Pat. No. 3,956,667). In such flat image display devices, i.e., image display devices using a flat picture screen, a fine-grain luminescent screen with a high light efficiency is required. Plasma in such devices functions as the actual cathode, from which an electron beam is drawn for each luminescent point to be excited via a perforated control panel having a matrix control. In comparison to a classic cathode beam image tube, where a single electron beam must reach all luminescent points, which, with typical large image tube dimensions, requires very high beam acceleration, the electron beams in gas discharge display devices are relatively low energy beams. Further, due to the flat screen structure and the gaseous atmosphere in such devices, high acceleration voltages are not possible. Therefore, it is extremely important for luminescent screens in flat image display devices to obtain a high light efficiency after conversion of electron energy into light or after light generation from the luminescent points or phosphor dots via, for example, impingement on such points or dots by ultraviolet radiation.
For improving light efficiency in classic cathode ray tubes, it is known to coat the luminescent material points or dots on their inner surfaces (i.e., away from the viewing surface), with a mirroring or reflecting metal layer, for example as described in U.S. Pat. No. 3,858,083. In this manner, light emitted by excited luminescent particles toward the back or rear of a so-called luminescent dot is largely reflected by this metal layer and added to the light emitted toward the front. However, the exciting electrodes are also weakened by this metal layer. Such partial electron deenergization is not overly detrimental for high-energy cathode beam electrons but with low-energy electrons used in flat plasma display devices, the use of such metal mirrors is detrimental. Nevertheless, such metal coatings would be desirable for luminescent material layers at the individual luminescent points thereof, not only for it reflective effect but also as a protective means against mechanical loads, particularly given the structure of a flat picture screen where spacer mounts or members are required between the luminescent screen and a perforated control plate. However, in order to minimize the de-energization effect of a metal layer, it would have to be so thin that it could no longer function as a protection means.
It is also known in classic cathode ray tubes to isolate individual luminescent points or dots from each other by opaque borders, for example from the earlier referenced U.S. Pat. No. 3,858,083. In conjunction with flat image display devices, this "black bordering" effect is described and claimed in commonly owned, co-pending U.S. application Ser. No. 012,348 filed Feb. 15, 1979, now U.S. Pat. No. 4,243,735. Such an opaque or black bordering layer about individual luminescent point surfaces is referred to herein and in the claims as "contrasting border layer". Generally, prior art contrasting border layer are comprised of granular glass solder or metal-organo compounds which are subjected to a heating process. When viewed in the light of the required spacer mounts or members for flat image display devices, such contrasting border layers are unsuitable as supports for the spacer mounts because the granular nature thereof cannot sufficiently resist mechanical loads.